docs: def _check_result(self, result):
      """ Check that the check returned a well formed result:
            a tuple (<Status>, message)
  
          A boolean Status is allowd and will be transformed to:
          True <Status: PASS>, False <Status: FAIL>
  
         Checks will be implemented by other parties. This is to
         help implementors creating good checks, to spot erroneous
         implementations early and to make it easier to handle
         the results tuple.
      """
      if not isinstance(result, tuple):
        return (FAIL, APIViolationError(
          'Result must be a tuple but '
          'it is {}.'.format(type(result)), result))
  
      if len(result) != 2:
        return (FAIL, APIViolationError(
          'Result must have 2 items, but it '
          'has {}.'.format(len(result)), result))
  
      status, message = result
      # Allow booleans, but there's no way to issue a WARNING
      if isinstance(status, bool):
        # normalize
        status = PASS if status else FAIL
        result = (status, message)
  
      if not isinstance(status, Status):
        return (FAIL, APIViolationError(
          'Result item `status` must be an instance of '
          'Status, but it is {} a {}.'.format(status, type(status)), result))
  
      return result